Variable valve activation (VVA) mechanisms for internal combustion engines are well known. It is known to be desirable to lower the lift, or even to provide no lift at all, of one or more valves of a multiple-cylinder engine, during periods of light engine load. Such deactivation or cam profile switching can substantially improve fuel efficiency.
Various approaches are known in the prior art for changing the lift of valves in a running engine. One known approach is to provide a latching mechanism in the hydraulic lash adjuster (HLA) pivot end of a rocker arm cam follower, opposite from the valve-actuating end, which locks and unlocks the valve actuator portion from the follower body. The cam follower mechanism is latchable by a lock pin whose motion typically is governed in a latching direction by application of pressurized engine oil and in an unlatching direction by a return spring. The lock pin is disposed as a piston in a smooth bore and is retained therein by a plug pressed into the end of the bore.
A prior art cylindrical plug serves to seal the prior art smooth bore, thus forming a hydraulic chamber between itself and an end of the lock pin. However, this prior art plug arrangement suffers from at least two shortcomings.
First, the axial rest position of the lock pin within the bore is not well defined by the manufacturing process because the distance from the inner end of the plug to the cam-actuated follower to be latched by the lock pin is not controlled. Further, and because of this, the installed compression of the return spring is not controlled, resulting in variability in required oil pressure for actuation among the population of rocker arm assemblies.
Second, two-step rocker arm assemblies are known to experience variation in actuation timing which can result in violent rejection of the lock pin when the lock pin is not fully engaged with the cam-actuated follower at the start of a lift event. Such rejection is known in the trade as a premature lock pin ejection. Such ejection can cause an instantaneous buildup in hydraulic pressure against the plug as the lock pin direction of travel is abruptly reversed, which can be sufficient to blow the plug from the bore, resulting in permanent failure of the rocker arm assembly (permanent lost motion of the rocker arm assembly and non-opening of the associated engine valve).
Third, some prior art two-step rocker arm assemblies include a retaining ring within the bore and the hydraulic chamber specifically to define the intended stroke length of the lock pin, which adds to the cost and complexity of manufacturing the rocker arm assembly.
What is needed in the art is an improved method and apparatus for forming a lock pin sub-assembly for a two-step rocker arm assembly wherein a bore plug cannot be blown out by a premature lock pin rejection; wherein a reliable hydraulic seal of the lock pin bore is formed; and wherein the position of the lock pin in its bore and the consequent compression of a return spring are fixed and controlled to a predetermined standard during manufacture of the rocker arm assembly.
It is a principal object of the present invention to provide a two-step rocker arm assembly having a highly precise response characteristic among members of a manufactured population of two-step rocker arm assemblies.
It is a further object of the invention to provide a simplified variable valve lift apparatus wherein manufacturing assembly is simplified and cost is reduced.
It is a still further object of the present invention to provide a two-step rocker arm assembly wherein a bore-sealing plug of a lock pin assembly cannot be expelled from the bore by a premature lock pin ejection of the rocker arm assembly during use.